Preheat oven to 350°F. Use 1 tbsp butter to grease soufflé pan. Cook the broccoli (9min with water, covered, in microwave) and blend or cut until very finely chopped (not puréed). In thick pan, melt remaining butter and add flour, mixing constantly. Add milk to form a thick sauce. Remove from heat and stir in cheese and broccoli. In separate bowl, whisk yolks. Add the warm mixture a little at a time, all the while whisking. Beat eggs until bubbly. Add salt and beat until stiff peaks form. Fold egg whites into the cheese sauce in two batches. Pour into dish and bake 50min.